Grease pencil animation tweening
Animating sketches which were created using Grease Pencil generally is being done with Dope Sheet Editor by creating / editing sketches for each frame of animation.

Animating with Dope Sheet

After first GP pencil layer is drawn, open Dope Sheet Editor and switch it to Grease Pencil mode. 



Once opened you'll see several (depending on amount of GP layers in your scene) channels with keyframe/-s.  From wiki:


  The DopeSheet should now display a few “channels” with some “keyframes” on them. These “channels” are the layers, and the “keyframes” are the frames at which the layer has a sketch defined. 


And because you have one frame created, go to the next frame and edit sketch as desired. Keep in mind that this is 1 frame of animation, so changes should be tiny. Continue creating frames as needed; very useful tools for making this easier are:


Enable Editing Strokes button in the 3D View > Toolshelf (lets edit already drawn strokes);
proportional editing (simple mode and Connected one for editing only current strokes);
grabbing, scaling, rotating.
duplicating sketches from one frame to another - select frame in Dope Sheet and in the window header choose Frame > Duplicate Keyframes; copy of the sketch will be created and placed atop of the existing one. Grab it where desired. 


It's possible to preview animation at any time by pressing Alt+A. Once there is desired amount of keyframes the animation is ready:



The downside of this method is that you should create each frame by hand. All the color information, thickness of the stroke etc. will be preserved though.

See How to sketch animation in Blender

Converting Grease Pencil and animating with Shape Keys

This way is completely based on using Join as Shapes. It allows you to morph between two objects simply joining them with using this option. However, the objects must have the same vertex count (which can be quite difficult for Grease Pencil conversion). Also it doesn't provide any flexible way to preserve stroke thickness and color as first way does.


Sketch 1-st frame. There can be either 1 GP layer contaning both frames or 2 different layers.
Sketch last frame.
Place selection on the first keyframe in Dope Sheet (if sketches are stored as keyframes in Dope Sheet) or choose desired Grease Pencil layer (if sketches are stored on different GP layers).
In the Toolshelf choose Grease Pencil > Convert. Choose Path or Bezier Curve. In the options of the last operator uncheck Link Strokes.



This will make end of the strokes not to be connected and will preserve their overall shape.
After converting second sketch select both of them, create Basis shape key for it and choose Join as Shapes from dropdown menu in Shape Keys panel.




The disadvantages are that drawing thickness and color aren't saved. Hence this way, inspite of providing possibility to not draw middle frames of animation, is less flexible.
Although, you can preserve data about how fast the strokes were drawn which could be used for creating Evaluate Time F-Curve and reconstructing the thickness of strokes. The Timing mode options let the curves have different thickness (more complex way).